Vibration of the heart valves generates heart sounds. The valves are forced into vibration at the instants of the valves' closure, caused by abrupt increase of the blood pressure gradient. A first heart sound (S1) is generated at the end of diastole, while a second heart sound (S2) is produced at the end of the systole. The sounds are transmitted through the body and can be heard on the body surface by use of the stethoscope, or be recorded using a sound transducer. Although heart sounds have been recorded in the past, they have not been used to automatically determine heart rate. The major problem being that for each heart beat there are four sounds generated by valve closure, typically grouped in pairs.